(1) Field of the Invention
The present invention relates to an air filtration assembly for use in filtering air containing toxic contaminants discharged from a contaminated air source, and in particular to a filtration assembly that can be replaced without the escape of contaminated air.
(2) Description of the Prior Art
Various processes and activities, whether of an industrial or research nature, have the potential to discharge toxic contaminants into the air within the work area. “Toxic contaminants” or “contaminants” as used herein includes numerous types of chemical and biological materials that can be harmful to humans, even in small quantities. Filtration of the air to remove such contaminants is essential. Therefore, most contaminant generating processes include a means for conveying contaminated air through a filtration system, normally including one or more particulate filters, e.g., HEPA (high efficiency particulate air) filters, that are capable of removing the contaminants, including very small toxic particles.
Preferably, the system is designed so that the filter and its support housing can be replaced without discharging contaminated air into the surrounding area. This result is commonly achieved with the use of an assembly known as a bag-in, bag-out filter assembly. When using this assembly, the filter, including its surrounding frame, is enclosed within an in-line conduit downstream of the contaminated air source and upstream of the discharge opening. The filter is normally inserted into the conduit behind a closeable opening in the conduit wall. The conduit opening usually includes a peripheral mounting flange. The mouth of a plastic bag sized to receive the filter assembly is mounted onto the flange over the opening.
When replacement of the filter is required, the operator withdraws the contaminated filter from the housing and into the bag. The bag is then tied off between the contaminated filter and the housing opening at two spaced locations. The operator then severs the bag between the locations, enclosing the contaminated filter within the bag and leaving a stub of the bag upper end still attached to the flange and extending across the opening. Alternatively, the bag may be sealed with a heated sealing device and the bag cut between the upper and lower edges of the seal.
The operator then attaches a second bag containing a new filter assembly onto the flange over the stub and then works through the second bag to detach the stub from the flange. The new filter assembly is then inserted into the opening, with the stub remaining in the bag.
Obviously, this is a time consuming and cumbersome procedure. More importantly, however, this procedure leaves the conduit without a filter from the time that the contaminated filter is withdrawn until the new filter is inserted. During this time, contaminated unfiltered air is conveyed through the conduit and discharged downstream creating a considerable risk in the case of some contaminants. In order to avoid this problem and the concomitant risk, such prior art systems often include a parallel filter that is used when the first filter is being replaced. This parallel filter system adds considerably to the cost of the system and operating expense, since two filters now have to be changed.
What is needed is a filter assembly and method that permits a quicker and simpler replacement of contaminated filters, a system incorporating this assembly to avoid the discharge of contaminated air during filter changes, and a method for implementing this filter change with the filter assembly.